A hairpin should begin from an established dynamic, not from silence

A crescendo/diminuendo wedge is read relative to the dynamic level it starts FROM; a hairpin with no established dynamic anywhere before its start point has no baseline to swell from or taper to, which is ambiguous rather than merely informal (contrast with expr-hairpin-no-terminal-dynamic, which covers the destination end).

A hairpin `{ from: 0, to: 8, type: "cresc" }` opens a piece with no dynamic ever stated on note 0 or any note before it — the crescendo has no baseline level to grow from, so a performer cannot judge how much louder to get.

Gould, Behind Bars, ch. Dynamics — hairpins read as a change relative to a stated level; Ross, The Art of Music Engraving, ch. Dynamic Markings, likewise presents every worked hairpin example starting from an already-stated dynamic (commonly the opening dynamic of the piece/section).

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.